This invention relates in general to optical non-reciprocal devices and in particular to an optical non-reciprocal device having improved performance at different temperatures and wavelengths of the light.
The use of fiberoptics in communications has developed rapidly in recent years. With such progress, new problems have arisen. Thus, when a light source transmits light through an optical fiber to another optical device, the light transmitted through the optical fiber will be reflected by an end face or other part of the optical device at the other end so that such reflected light returns to the light source. Multiple reflections on the end faces of optical fibers or other optical devices can cause echoes. These effects adversely affect the performance of the source and compromise the information communicated in the fiber.
Various optical isolators and non-reciprocal devices have been developed to overcome the problem of light reflections and echoes described above. In one type of prior art optical isolators or optical non-reciprocal devices, such device can be used only at such points where the linearity of the polarized light is maintained in the optical fibers. Furthermore, the performance of these conventional devices is severely degraded by variations in temperature and wavelength of the light through. Such prior art devices are therefore not entirely satisfactory.
In view of the drawbacks of the above-described prior art optical isolators and non-reciprocal devices, polarization independent isolators and couplers have also been developed. Described below are two optical devices which are polarization independent. Both devices employ birefringent crystals and Faraday rotators. It is known that when an anisotropic crystal is cut in a certain manner, the crystal will cause the component of light in a particular polarization direction to be diverted into a different path upon passing through the crystal. The direction of divergence is known as the walk off direction of the crystal. It is also well known that birefringent crystals will only cause the component of light in the polarization plane parallel to the walk off direction to walk off and does not affect the component with polarization in the plane perpendicular to the walk off direction. Faraday rotators are also well known to those skilled in the art as a non-reciprocal rotator and is therefore different from reciprocal rotators such as half-wave plates.
Uchida et al., in U.S. Pat. No. 4,178,073, discloses a polarization independent optical isolator employing birefringent crystals and a Faraday rotator. In the forward direction, the light passes through the following elements sequentially a first birefringent crystal, a Faraday rotator, a half-wave plate, and a second birefringent crystal. In the forward direction, the first birefringent crystal in the optical isolator divides an incident light beam into an ordinary ray and an extraordinary ray which are then synthesized into one beam when emerging from the second birefringent crystal in the isolator. In the reverse direction, however, while an incident light beam is divided again into an ordinary and an extraordinary ray, but the two rays are not synthesized upon emerging from the isolator in the reverse direction. Thus, when light passes from a first to a second optical fiber, if the optical isolator is inserted between the two fibers, the isolator will pass light in the forward direction between the two fibers but will prevent light reflections from the second fiber from being transmitted in the reverse direction to the the first fiber. In U.S. Pat. No. 4,239,329, Matsumoto discloses polarization independent optical nonreciprocal devices which achieve essentially the same result as that of Uchida et al.
While the devices disclosed by Matsumoto and Uchida et al. are advantageous over polarization dependent prior art devices, the performance of Uchida et al. and Matsumoto's devices are severely degraded by variations in temperature and wavelength of the light. It is therefore desirable to provide improved optical non-reciprocal devices which are insensitive to variations in temperature and wavelengths of the light.